1. Field of the Invention
The present invention relates to a bus instrument and method for a two-wire instrumentation bus, and more particularly, to a bus instrument and method for predictively limiting power consumption in a two-wire instrumentation bus.
2. Statement of the Problem
Bus loops are commonly used to connect various instruments, such as in industrial settings, for example. The bus loop can provide electrical power to the bus instruments. The bus loop can enable communications between a bus instrument and an external device(s). For example, a bus loop is commonly used for reporting of bus instrument measurements and further may enable control of the bus instrument.
One bus loop protocol is a two-wire bus protocol, sometimes referred to as a 4-20 milliAmp (mA) bus because of inherent power limitations. This bus protocol may be employed for the simplicity of connecting instruments to the bus using only two wires, where the two wires provide both electrical power and electronic communications. This bus protocol may be used in hazardous or explosive environments, for example, where electrical power is limited for reasons of safety.
Communication of measurement values in common two-wire bus protocols comprises controlling and varying the current draw to a predetermined range, such as between 4 and 20 mA. In this bus protocol, a zero flow condition is denoted by controlling the loop current in the bus loop to be 4 mA. A bus loop current of less than 4 mA is not a valid measurement according to the two-wire bus protocol, and can comprise a power-up phase of the bus instrument or some other manner of signaling. Likewise, a maximum flow amount can result in the bus instrument controlling the bus loop current to be about 20 mA.
A host system is connected to the two-wire bus loop and provides the regulated electrical power and receives the communication signaling from all connected bus instruments. The host system translates the current amount (i.e., the measurement value) and passes the measurement value to an external device, such as a monitoring computer.
The limited electrical current and the limited electrical power can be problematic. The bus instrument must operate accurately and reliably without exceeding the current limitations. Increased power demand in the bus instrument can cause the electrical current requirements of the instrument to exceed an amount prescribed by an applicable protocol. Further, in conditions of minimal flow, a two-wire bus instrument cannot consume more than 4 mA of electrical current. This low current level may be problematic and may not be enough electrical current to operate the bus instrument.
In some embodiments, if the power consumption reaches or exceeds an available power limit, then an error condition may result. The error condition in some embodiments may result in faulty or unreliable operation of the bus instrument. The error condition in some embodiments may result in a reset of a processor or processors in the bus instrument.
Therefore, it is desirable that the power consumption of the bus instrument be kept below the allowable power limit if at all possible.